1. Field of the Invention
The present invention relates to a process for producing a polyester. More particularly, it relates to a process for economically producing a polyester using butanediol as a diol component in a simple step.
2. Description of the Prior Art
Heretofore, it has been known that 1,4-butanediol can be used as the diol component in a process for producing a polyester. The resulting product has been widely used because these polyesters have superior properties such as moldability, surface characteristics, size accuracy and the like. Among such polyesters, the most useful one is polybutyleneterephthalate which is effectively used as an engineering plastic for decorative components in electrical apparatus, machines, cars, sporting goods, interior decorative goods and the like.
In order to produce polyesters derived from 1,4-butanediol such as polybutyleneterephthalate, the reaction of one mole of terephthalic acid and/or a dialkyl ester with 1.1-2.2 moles of 1,4-butanediol is conducted at about 150.degree.-230.degree. C. for about 0.5-5 hours to produce bis(hydroxybutyl) terephthalate and oligomers thereof. Thereafter, the reaction is continued, heating at a temperature of about 230.degree.-260.degree. C. under a gradually reduced pressure of about 0.05-10 mmHg, until a polyester having a suitable degree of polymerization is produced. Depending upon the progress of the polycondensation, an excess of 1,4-butanediol is distilled out from the reaction system. It is quite important to recover and to reuse the distilled 1,4-butanediol mixture in the polycondensation system in order to lower the cost of the production of the polyester.
Customarily, the distilled 1,4-butanediol mixture is condensed, collected and purified by other refining processes such as distillation. However, 1,4-butanediol has low thermal stability, decomposing to tetrahydrofuran and water so that the refining step has low efficiency. Accordingly, it is necessary to conduct a pretreatment of the distilled 1,4-butanediol mixture such as by neutralization of the acidic impurities, with an alkali in order to prevent the decomposition of 1,4-butanediol to tetrahydrofuran. Consequently, there continues to exist a need for a more efficient, simple process for reusing the 1,4-butanediol in this reaction system.